In an image forming apparatus employing an electro-photographic method, after an electrostatic latent image is formed on a photoconductor by laser light rays, the electrostatic latent image is developed to be a toner image using toner particles, then the toner image is directly transferred onto a sheet, or the toner image is transferred onto an intermediate transfer member, which is then transferred onto the sheet. Next, the toner image is fixed on the sheet by pressure and heating by a fixing apparatus. The fixing apparatus incorporates a fixing roller having a halogen lamp as a heat source, and a pressure applying roller to press against the fixing roller, wherein the sheet carrying the toner image is introduced through a pressure-contact region between the heated fixing roller and the pressure applying roller, and thereby the toner particles are melted and fixed onto the sheet.
The fixing roller, which fixes the toner image, incorporates an elastic layer and a releasing layer. In case of full-color image fixing, an amount of employed toner is very large, and in order to apply effective pressure onto the sheet carrying the toner image, it is necessary that the surface area of the pressure-contact region is enlarged, and that the time interval to sufficiently heat the toner is obtained. Since the adhering force is increased between the sheet and the fixing roller after the toner image is fixed, a separating claw is generally used to separate the sheet from the fixing roller.
However, there is a risk of the claw damaging the fixing roller. Further, since the amount of toner is very large, the amount of wax in the toner is also large, whereby wax is adhered to the separating claw. Scarring of the fixing roller or adhered wax results in the defected images or dirty images. Accordingly it is preferable that a separating claw should not be used to separate the image carrying sheet in the image forming apparatus.
As a sheet separating method using no claw, there is a curvature separating method. In the curvature separating method, while the fixing roller widely changes the conveyance direction of the sheet from the exit point of the separation section by its own curvature, the sheet keeps straightness due to its stiffness, whereby the sheet can be easily separated from the surface of the fixing roller.
However in such a fixing apparatus, since an elastic layer is provided on a periphery of the fixing roller, heat capacity of the fixing roller is so large that when the apparatus is electrically activated or turned to operation mode from its energy saving mode, the fixing apparatus consumes greater electrical power and requires a long start-up time to heat up the fixing roller to the temperature required to conduct the fixing operation. Further, during the standby condition until the next image formation starts, the heat is transmitted to the pressure applying roller through the pressure-contact region, therefore, in order to keep the fixing roller at the essential temperature for the fixing operation, the fixing roller needs to be continuously heated, which results in a problem of energy loss.
To overcome these problems, Japanese Laid-Open Patent Publication No. H8-262893 discloses a fixing apparatus in which the start-up time is shortened.
In this fixing apparatus, an endless belt, instead of the pressure applying roller, is used for reducing the heat capacity. That is, an elastic pressing member and a belt guide are arranged parallel to the fixing roller, and the endless belt is provided outside the elastic pressing member and the belt guide. The endless belt presses against the surface of the fixing roller by the elastic pressure applying member, and the endless belt is driven by the rotation of the fixing roller.
In this fixing apparatus, since the elastic pressing member presses the endless belt onto the fixing roller, the thickness of the elastic layer of the fixing roller is reduced, and thereby a fixing roller having a smaller diameter can be used. Further, the width of a pressure applying pad is increased in the rotation direction of the fixing roller so that the area of the pressure-contact region is increased, which improves the efficiency of the fixing apparatus. Yet further, the thickness of the elastic layer of the fixing roller is reduced so that the heat capacity of the fixing roller is reduced, which can shorten the start-up time. In addition, the thickness of the endless belt is reduced so that any heat escaping from the fixing roller through the pressure-contact region is also reduced, and thereby there is no need to turn on the heating lamp during the waiting time until the next image formation starts, which is very effective to save energy.
However, in this fixing apparatus, the problem described below exists. That is, in the above-described fixing apparatus, since the endless belt is in pressure-contact with the small diameter fixing roller by the elastic pressure applying member, the endless belt at the pressure-contact region is concaved while going over the curved surface of the fixing roller. Further, after the endless belt passes through the pressure-contact region, that is, when the endless belt reaches downstream of the pressure-contact region, the endless belt moves along the surface of the fixing roller.
Accordingly, at the exit of the pressure-contact region, force is generated to curve the sheet over the curvature of the fixing roller, which prevents the separation of the sheet from the fixing roller, or any toner separated from the fixing roller may again adhere onto the fixing roller, which may result in the sheet being wound around the fixing roller.
Accordingly, Japanese Laid-Open Patent Publication No. H11-2987 discloses a pressure applying pad shown in FIGS. 8 and 9. FIG. 8 shows a structure of relevant parts of the conventional fixing apparatus, while FIG. 9 shows an enlargement of a part of FIG. 8.
Fixing roller 51 is rotatably supported around the central axis, and endless belt 52 is provided under fixing roller 51. Within endless belt 52, pressure applying pad 53 and belt guide 54 are integrally supported parallel to the axis of fixing roller 51. Around pressure applying pad 53 and belt guide 54, endless belt 52 is arranged so that endless belt 52 can move under a non-tension condition in the direction of rotation. Endless belt 52 comes into pressure-contact with fixing roller 51 by pressure applying pad 53. As shown in FIG. 9, on pressure-contact region Z, fixing roller 51 and pressure applying pad 53 come into pressure-contact with pressure applying member 64, along length W in the rotational direction of fixing roller 51.
Fixing roller 51 is formed of coating layer 62, including base layer 62a and separating layer 62b, formed on the surface of cylindrical steel base 61. Elastic base layer 62a is made of silicon rubber, and separating layer 62b is dip-coated Viton rubber.
Halogen lamp 57 is provided as a heat source in the space of steel base 61 of fixing roller 51. Temperature sensor 58 detects any change of temperature of the surface of fixing roller 51, and a feedback system, which is not illustrated, functions to control halogen lamp 57, based on signals from temperature sensor 58, so that the surface temperature of fixing roller 51 is controlled within a predetermined temperature range.
Endless belt 52 is formed of thermosetting polyimide, at a thickness of less than 100 μm, and the periphery surface is covered with 30 μm of fluorocarbon resin as a separating layer. The inner length of endless belt 52 is set to be slightly longer than the length totally including pressure applying pad 53 and belt guide 54.
Pressure applying pad 53 includes elastic pressure applying member 64 made of silicon rubber, and separation member 65 which is on the downstream of and adjacent to the pressure-contact region of pressure applying member 64, both of which are arranged on supporting member 56. Supporting member 56 is supported on belt guide 54. The surface of pressure applying member 64 is covered with low friction film 66. Separation member 65 is structured of a metallic material in bar. Low friction film 66 enables endless belt 52 to move easily, and includes fiber glass sheet impregnated with fluorocarbon resin.
Now, the fixing procedure of the fixing apparatus will be explained. Sheet P, on which color toner image “t” has been transferred by a transfer apparatus which is not illustrated, is conveyed to pressure-contact region Z between fixing roller 51 and endless belt 53. Fixing roller 51 is rotated by an unillustrated drive motor in direction “a” in FIG. 8. Endless belt 52 is driven by fixing roller 51 to move around in direction “b”. In pressure-contact region Z, endless belt 52 is curved in the same degree of curvature as the contact periphery of fixing roller 51. Sheet P, conveyed into pressure-contact region Z, is heated by fixing roller 51 which itself is heated by halogen lamp 57, and is pressed by pressure applying pad 53 through endless belt 52, whereby toner image “t” is fixed onto sheet P.
Sheet P, which passed through pressure-contact region Z, tends straighten itself due to its inherent elastic force, but sheet P might also adhere and be wound around fixing roller 51 at the exit of pressure-contact region Z due to the stickiness of the melted toner. Consequently, separation member 65 is provided to improve the curvature separation ability of the sheet.
That is, since fixing roller 51 is pressed by separation member 65 to have an elastic deformation, concave section of depth δ is generated, therefore, the curvature of a partial section of fixing roller 51 becomes large. Due to this, endless belt 52 is curved with large curvature along the surface of separation member 65 at the downstream of pressure-contact region Z, and endless belt 52 is driven in the direction separating from fixing roller 51. As a result, sheet P is also conveyed in the direction separating from fixing roller 51 at the exit of pressure-contact region Z, which further improves the effect of the curvature separation. By this structure, though a separating claw is not used, very thin sheets P can be separated from fixing roller P without trouble.
However, the fixing apparatus described in Japanese Laid-Open Patent Publication No. H11-2987 exhibits the shortcoming below. Sheet P is of varying thickness and quality. For example, in a case of a thick sheet, the elastic force of the sheet is so large that the sheet separates from fixing roller 51 easily, causing fixing of the toner image to deteriorate. It is because as the sheet thickness increases so does the heat capacity of the sheet, which takes a longer time to melt the toner particles. On the other hand, in the case of too thin a sheet, the heat capacity is so small that fixing can be conducted satisfactorily, but the elasticity of the thin sheet is so small that the separation from the fixing roller 51 becomes a major problem. In addition, in the fixing apparatus described in Japanese Laid-Open Patent Publication No. H11-2987, the position and the pressure at which pressure applying member 64 and separation member 65 come into pressure contact with fixing roller 51 is constant, and cannot vary in accordance with the characteristics of sheet P.
Additionally, endless belt 52 does not always come into pressure-contact with fixing roller 51, but endless belt 52 goes down to separate from fixing roller 51 at a clearance of 1-2 mm, while awaiting the fixing operation. Because if endless belt 52 is always in pressure-contact with fixing roller, fixing roller 51 and endless belt 52 tend to deform and deteriorate so that their operating life shortens. For this reason, they are separated from each other while awaiting the fixing operation, and longer function can be attained from them. When image formation starts, pressure applying member 64 goes up, and brings endless belt 52 into pressure contact with fixing roller 51. It is possible to adjust the contact pressure by changing the raising amount of pressure applying member 64, but in this method, the applying pressure of pressure applying pad 53 and separation member 65 change simultaneously in the same direction, which can not allow adjustment of the applied pressure in accordance with the sheet characteristics.